Trypanosomes of the brucei subgroup infect a wide variety of species but only cause severe disease in some hosts (e.g. African sleeping sickness in man). Because the parasite is extracellular at all stages of infection, an understanding of the interaction of the parasite's surface with its extracellular milieu (plasma proteins, red blood cells, brain tissue, etc.) is important in elucidating the pathogenesis of the disease as well as the survival tactics of the parasite. The surface of Trypanosoma brucei is densely covered with a single surface glycoprotein (VSG). Although much is known about the structure of VSG adn its unique C-terminal glycolipid, the detailed aspects of its attachment to the cell surface remain poorly understood. The mechanism of release of this protein from the cell, which is required at certain stages of its life cycle, has not been defined. Our studies are based on the model that attachment of VSG to the cell surface requires not only hydrophobic interactions between the glycolipid anchor and plasma membrane phospholipids but also electrostatic interactions between basic amino acids at the C-terminal and of VSG and acidic phospholipid head-groups in the plasma membrane. Our studies to date have shown that VSG with its glycolipid anchor (mfVSG) can transfer from trypanosomes to another lipid environment (e.g. the plasma membrane of erythrocytes or tissue culture cells, liposomes, lipid monolayers). These findings will be pursued in this proposal which focuses on further studies to determine the biochemical and biophysical parameters of mfVSG interaction with lipids. Both model membranes (liposomes, lipid monolayers) and cell membranes (erythrocytes) will be used as acceptor membranes. While experiments can be performed with living trypanosomes, in others the reconstitution of mfVSG into liposomes or monolayers will be required. VArious parameters of the transfer process (ion and pH dependence) as well as possible resultant functional changes (permeability) in the acceptor membrane will be examined. We will determine in more detail the kinetics of intermembrane transfer of mfVSG from trypanosomes or VSG-containing reconstituted liposomes to acceptor membranes. The effect of different lipid environments on the susceptibility of mfVSG to phospholipase C and D will be examined. These studies are a direct continuation of experiments performed during the previous grant period adn may suggest a mechanism for the ability of trypanosomes to induce pathological changes in the host.